The present invention relates generally to wireless telecommunication systems, and more specifically to reliably estimating the power of a signal received in a receiver operating according to a CDMA (or similar) standard, even if the signal includes a significant amount of interference. The present invention is especially useful when the receiver uses the economically advantageous direct-conversion architecture because the direct-conversion open-loop power control circuit filters out less interference when compared to a standard super-heterodyne receiver.
Wireless telecommunication technology has revolutionized the way people communicate over a distance. From the inception of modern telecommunication systems in the nineteenth century, virtually all such systems, save those relying on line-of-sight, are operated over a wire. Using a pattern of electrical signals transmitted from one end of the wire to the other, devices at each end have been able to establish effective and reliable communications. More accurately, to traverse long distances the electrical signals traveled a number of wires interconnected by switches to form a network. The switches permit almost any desired combination of wires to be connected so that any device may connect and communicate with any other, so long as they are both linked to the network and not otherwise occupied. This type of network, which started as telegraph and became telephone, is now often referred to as the plain-old telephone system (POTS). Similar private or limited-access networks are also in current use. Such networks provide a reliable vehicle for communications traffic, including voice and data, between devices such as telephones and computers. The main disadvantage of the POTS and similar wire line systems is that these devices must be located at a network-access point to which they are physically connected, and cannot be relocated during an ongoing communication session. In addition, whatever network resources are employed to establish the continuous connection between the calling party and the called party must remain so employed for the duration of the session-even at intervals where no actual communication is taking place.
Wireless communications, which rely chiefly on radio waves, address both of these concerns. Although the first radio transmission took place years ago, it is only in the relatively recent past that the technology has progressed to allow widespread use of wireless communication by the general population. A great many people now subscribe to mobile as well as (or instead of) wireline service. Typically, subscribers will have one or more mobile stations capable of radio communication with network transceivers located through the network coverage area. The mobile station itself includes a transmitter and a receiver, and will usually be a telephone, computer, personal digital assistant (PDA), or similar device. Naturally, there is no requirement that the mobile station actually be movable, and as used herein the term xe2x80x9cmobile stationxe2x80x9d will apply to these devices and any others similarly capable of radio communication with a wireless network.
FIG. 1 is a block diagram illustrating selected components of a typical wireless network 100, as might be used in communicating with mobile stations practicing received-power estimation according to the present invention. The network 100 includes a plurality of base service stations (BSSs), here BSS 105, BSS 110, and BSS 115. Although only three BSSs are shown, however, in an actual network there would be a great many. Each BSS includes a base station transceiver (BST) and a base station controller (BSC). In FIG. 1, for example, BST 106 and BSC 107 are included in BSS 105; BST 111 and BSC 112 in BSS 110; and BST 116 and BSC 117 in BSS 115. Alternately, a single BSC may control a number of BSTs. The coverage area of network 100 is divided into a number of cells, each having a BST (and perhaps a BSC). For purposes of illustration, three cells are enumerated in FIG. 1 as cell 104, cell 109, and cell 114. A typical network has many such cells that, unlike these illustrated in FIG. 1, may vary in size and overlap each other.
Individual subscribers use mobile stations, such as mobile station (MS) 130, to communicate with and through the network, usually through the BST covering the cell in which the mobile station is currently located, or the BST of a neighboring cell. In this way each radio frequency being used for mobile station to base station communication may be re-used for similar communications in another cell, so long as the other cell is sufficiently far away to prevent the signals from crossing or interfering with one another. Naturally, transmission power is limited so that transmissions taking place in one cell do not reach (at significant levels) another cell where the same frequency is being used. In the embodiment of FIG. 1, MS 130 is in communication with BSS 115 through a radio-frequency (RF) link 135. RF link 135 includes a forward link 136, which carries transmissions from BST 116 to MS 130, and a reverse link 137, which the MS 130 uses for transmitting to BST 116. Reverse link 137 may be used for both traffic (actual communication) and control signaling. The same is true of forward link 136, which includes a pilot channel, a sync channel, and a paging channel. A pilot channel signal is transmitted continuously by each BST and is the means by which a mobile station locates a base station so that it can register (makes its presence known to the network) or establish a voice or data link for actual communication. The sync-channel message allows each mobile station to obtain frame synchronization of the CDMA signal. The paging channel is used for paging mobile stations to provide notice of incoming calls, and for similar messages.
Under ordinary conditions, MS 130 communicates only with BSS 115 while located in cell 114. As it travels toward, for example, cell 109, it detects that the pilot channel message from BSS 110 is getting stronger, and may establish communication with it as well. (The network may also allow for communication through the BSSs associated with neighboring cells where the closest BSS is available.) This redundancy enables continuity of communication as MS 130 relocates. When a mobile station travels from one cell to another, it switches from one BST to another in a process called xe2x80x9chand off.xe2x80x9d Ideally, communications with the second BST will be established before the link to the original BST is broken, in which case (called xe2x80x9csoft hand-offxe2x80x9d) the subscriber will perceive little or no break in the transmission.
BSSs may communicate directly with one another, such as BSS 105 and BSS 110 of FIG. 1, although they are all connected directly or indirectly through a mobile switching center (MSC) such as MSC 120. MSC 120 switches voice calls between mobile stations communicating with BSSs it is connected to, and switches calls to any other devices through the network (not shown) itself. Information concerning the mobile stations operating in the area is stored in a Visitor Location Register (VLR) database 122, which is connected to the MSC 120. MSC 120 may also include a packet-data switching node (PDSN) 124 that similarly switches packet data to and from more modern mobile stations.
As mentioned above, cellular telephony""s frequency re-use allows many more subscribers to be actively communicating than would otherwise be possible. That is, if all of the mobile stations in, for example, were each allocated their own frequency to use anywhere in a large metropolitan area, the available channels would soon be exhausted and busy signals would be very common. With frequency re-use, however, the same frequency channel being used by one mobile station may be assigned to another operating only a few miles away.
In addition, several multiple access schemes have been developed. Standard frequency-division multiple access (FDMA) simply divides the available transmission spectrum (bandwidth) into individual channels, and assigns these channels as they are needed for voice or data communications. In any such communication session, however, a significant amount of the time no actual transmission will be taking place; the channel is simply not need all of the time. It is assigned, however, for the duration of the session, meaning it cannot be used by another (local) subscriber. A multiple-access scheme permits users operating in the same or neighboring cells to share a frequency channel.
In a TDMA scheme, for example, each available frequency is divided up into time slots, with individual communication channels being defined by a particular time slot at a given frequency. If the number of time slots is kept low enough, almost any transmission may be sent without a delay perceivable to the subscriber. Using TDMA, many more simultaneous communications may take place within a given call. Of course, the sending station and the receiving station must be synchronized so that each used the appropriate time slot, meaning that the mobile stations must be somewhat more sophisticated. Nevertheless, a significant advantage is thereby gained when time-division multiple access (TDMA) is employed.
Another multiple access scheme is code-division multiple access (CDMA). CDMA does not involve dividing the available bandwidth into separate frequency channels at all, but instead uses a spread-spectrum transmission method. In a spread-spectrum system encoded transmissions are spread over the available transmission bandwidth using pseudorandom noise (PN) codes. Channelization is provided by additionally spreading the channel with one of 64 available waveforms called Walsh codes. Each mobile station establishing contact with a BSS is assigned a Walsh code, which is transmitted to the mobile station and used there to decode the received signal. Only the transmission intended for the particular mobile station may be received by it because its assigned Walsh code is (substantially) mutually orthogonal to other Walsh codes. Careful synchronization between the base station and each mobile station it is communicating with is necessary to ensure proper reception. Each BST continually transmits a sync message on the sync channel for this purpose. As with TDMA, however, the additional sophistication required in a CDMA system is justified by the relatively large number of subscribes that will be permitted to use the network simultaneously.
The CDMA multiple-access scheme currently operates according to a standard known as IS-95 standard, or according to its newer counterpart, IS-2000. The two standards are basically compatible, such that equipment constructed according to one may communicate with equipment constructed according to the other. IS-2000 (sometimes referred to as CDMA 2000), however, provides for the efficient transmission of data and multimedia content. The present invention may be advantageously employed with either IS-95 or IS-2000. Because it is not exclusively associated with either revision, the term xe2x80x9cCDMAxe2x80x9d will herein include both of them and any future versions.
Receivers operating according to the CDMA standard will be most advantageously used in accordance with the present invention because CDMA mobile stations employ variable transmission-power scheme. That is, the power level of separate transmissions may be raised or lowered, with the goal being that no transmission is over-powered. This both conserves mobile station power, which is often supplied by a battery, and minimizes cross-channel interference. Such power control is essential in any system, such as CDMA, where many subscribers are actually sharing the same frequency. There are several methods for controlling mobile-station transmission power. In open-loop control, the mobile station determines how much power to use for transmission by analyzing the power-level of a signal received from the base station. Typically, the base-station signal used is a calibration constant transmitted on the sync channel. The sync-channel signal power, of course, decreases with distance and so becomes weaker as the distance from the mobile station to the base station increases. Upon sensing this attenuation, the mobile station adjusts transmission power accordingly.
In one open-loop method of estimating the power-level of a received CDMA signal is to employ an analog automatic-gain-control (AGC) circuit is employed. As the received signal is processed, the AGC attempts to keep constant the power level of the signal as supplied to an analog-to-digital (A/D) converter used prior to demodulation. The gain value required to do this will vary with the power level of the signal actually received, and the former can therefore be used to estimate the latter. This method has been successfully employed, for example, in the traditionally-used super-heterodyne receiver, as described below.
FIG. 2 is a block diagram illustrating the open-loop power estimation components of a typical super-heterodyne receiver 200 of the prior art. The analog AGC loop 210, exemplary of those referred to above, is delineated by a broken line. Before passing through this loop, however, the received (RF) signal is amplified by a low-noise amplifier (LNA) 220. The amplified signal is sent to a mixer 225, where it is down-converted to an intermediate frequency (IF). As indicated in the xe2x80x9cLegendxe2x80x9d of FIG. 2, after mixing the signal includes a real part and an imaginary part. The complex IF signal is supplied to a surface-acoustic wave (SAW) filter 230, which substantially attenuates any interference associated with the received signal. The signal is then passed through a second mixer 235, which down-converts it to baseband. The baseband signal is then supplied to a baseband analog filter 240, which removes most, if not all, of the remaining interference that was previously associated with the received signal.
The filtered baseband signal is then supplied to a variable-gain amplifier 250 controlled by the analog AGC circuit 255 of the aforementioned AGC loop 210. The input for the analog AGC 255 is the output of A/D converter 260, which process the signal output from the variable-gain amplifier 250. Note that in this embodiment the AGC 255 also sets a gain value for the low-noise amplifier. It is the gain value supplied to the variable-gain amplifier, however, that is used by received-power estimation circuit 270 to estimate the power of the received signal. Received-signal power, in other words, is estimated by monitoring the analog gain-value necessary to maintain the output of variable-gain amplifier 250 to A/D converter 260 at a constant, predetermined level. The stronger the incoming signal, the less adjustment is necessary.
Note that in the super-heterodyne receiver circuitry 200 of FIG. 2, the IF-SAW filter 230 and the baseband analog filter 240 collectively remove most or substantially all of the originally present side-channel interference. This means that the gain values set by the AGC do not reflect also the power level associated with the interference. If this IF filtering is not done, the gain value may be less, causing received-power estimator 220 to over-estimate the received signal power.
So-called direct-conversion CDMA receivers are currently being developed, however, that for reasons of economy do not include an IF-SAW filter. Accurate power estimation according to the configuration of FIG. 2 is therefore not feasible. The higher level of interference remaining in the signal provided to the AGC makes the gain value an inaccurate power-estimation value. Needed, then, is a way to accurately estimate the power of a received RF signal. The present invention provides just such a device and method.
The present invention is directed to a device and method for estimating the power level of a received signal in a wireless telecom network, such as one constructed according to a CDMA standard. In one aspect, the present invention is a receiver circuit having a first AGC loop and a second AGC loop. In a preferred embodiment the first loop precedes the second and is an analog AGC loop, and the second is a digital AGC loop separated from the first by a digital filter. The system further includes a multiplier for multiplying the gain value set by the analog AGC with the gain value set by the digital AGC to produce a gain-value product. The logarithm of the gain-value product is then used to estimate CDMA power by applying it to an equation having pre-set parameters.
In another aspect, the present invention is a method for estimating the power level of a received CDMA signal, including the steps of amplifying the received signal, filtering the received signal through an analog baseband filter, and supplying the filtered signal to an analog AGC loop. The method further includes the steps of filtering the output of the AGC loop using a digital filter and providing the digitally-filtered output to a digital AGC loop. The linear gain value set by the analog AGC loop is the multiplied with the linear value of the digital-AGC-loop gain value to produce a gain-value product. Finally, the received signal power level is estimated by comparing the logarithm of the gain-value product to the power estimation curve associated with the receiver receiving the signal.